Hybrid systems for the application in vehicles are taught in a number of embodiments in the prior art. Common in all is that in the drive train at least two different prime movers are provided, through which the drive can occur selectively or jointly. At the same time, at least one of the prime movers is designed such that it is generally capable, in fuel cutoff operation and/or in the brake operation, of converting the mechanical energy into a different form of energy and feeding the energy to an accumulator unit and/or to a consumer. To do that, in particular, the second prime mover is designed in the form of an electrical machine suitable to feed the electric energy into an accumulator during generator operation. Such a hybrid system, for instance, is taught in the prior publication DE 103 10 831 A1, FIG. 30. This discloses a power transmission unit that is disposed between two prime movers and a downstream consumer in the form of a transmission unit. Furthermore, a device for disconnecting/connecting the power flow, in particular, in the form of a selectable clutch device is provided for selective coupling of the power transmission unit with one of the prime movers at least between this one prime mover and the power transmission unit. The clutch device is also designated as prime mover clutch or separating clutch. The second prime mover is designed as an electric machine in which the armature is connected non-rotatably with the power transmission unit. The power transmission unit comprises a hydrodynamic component and a device for at least partial bypass of the power flow via the hydrodynamic component. This is preferably done in the form of a selectable clutch device that is also designated as lock-up clutch and that enables bypassing the hydrodynamic component in power flow.
Such a start-up and power transmission unit formed by power transmission unit and selectable clutch device as well as a prime mover is operable in different modes. In particular, upon disconnection of the power flow between the prime mover and the power transmission unit when driving by means of the electric machine in the “electric drive” mode of operation, the selectable clutch device in the form of prime mover clutch is in the—open—position. However, a part of the operating medium and/or control medium is still contained in the pressure chamber allocated to the piston element, so that, owing to the connection of the piston element to the input of the power transmission unit, which, in this operating state, is concurrently driven via the electrical machine, a rotating ring for operating and/or controlling medium, in particular an oil ring, sets in. This acts on the selectable clutch device, in particular on the pressure-chamber-limiting face of the piston element and generates an axial force that in general is greater than the force that sets in on the piston face side towards the clutch. In the inconvenient case, this can lead to unintentional closure of the selectable clutch device and cause disadvantageous effects on drive responses, which, in particular, are characteristic of undesired dragging of connected elements coupled with the first clutch part.